Christian Nentwich has created BOX, a tool that
takes UML diagrams in XMI format and renders them in either SVG or Microsoft's
VML format.

Nentwich warns that "the state of the code is rather embarrassing and there
is no-one to maintain it, so I hesitated for quite a while." He also describes
the modelling tools required to use his code:

Since diagram information in XMI is (so far) proprietary, the tools
requires you to use the Unisys exporter for Rational Rose (or Select
Enterprise). It uses simple heuristics to restore the diagrams from the
rather poor info the exporter makes available.

In addition to the rendering functionality, Nentwich's code can produce
DTDs from UML models -- a topic attracting increasing interest and debate.